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  <div class="section" id="module-__future__">
<h1>28.10. <tt class="xref docutils literal"><span class="pre">__future__</span></tt> &#8212; Future statement definitions<a class="headerlink" href="#module-__future__" title="Permalink to this headline">¶</a></h1>
<p><tt class="xref docutils literal"><span class="pre">__future__</span></tt> is a real module, and serves three purposes:</p>
<ul class="simple">
<li>To avoid confusing existing tools that analyze import statements and expect to
find the modules they&#8217;re importing.</li>
<li>To ensure that future_statements run under releases prior to 2.1 at least
yield runtime exceptions (the import of <tt class="xref docutils literal"><span class="pre">__future__</span></tt> will fail, because
there was no module of that name prior to 2.1).</li>
<li>To document when incompatible changes were introduced, and when they will be
&#8212; or were &#8212; made mandatory.  This is a form of executable documentation, and
can be inspected programmatically via importing <tt class="xref docutils literal"><span class="pre">__future__</span></tt> and examining
its contents.</li>
</ul>
<p>Each statement in <tt class="docutils literal"><span class="pre">__future__.py</span></tt> is of the form:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">FeatureName</span> <span class="o">=</span> <span class="n">_Feature</span><span class="p">(</span><span class="n">OptionalRelease</span><span class="p">,</span> <span class="n">MandatoryRelease</span><span class="p">,</span>
                       <span class="n">CompilerFlag</span><span class="p">)</span>
</pre></div>
</div>
<p>where, normally, <em>OptionalRelease</em> is less than <em>MandatoryRelease</em>, and both are
5-tuples of the same form as <tt class="docutils literal"><span class="pre">sys.version_info</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="p">(</span><span class="n">PY_MAJOR_VERSION</span><span class="p">,</span> <span class="c"># the 2 in 2.1.0a3; an int</span>
 <span class="n">PY_MINOR_VERSION</span><span class="p">,</span> <span class="c"># the 1; an int</span>
 <span class="n">PY_MICRO_VERSION</span><span class="p">,</span> <span class="c"># the 0; an int</span>
 <span class="n">PY_RELEASE_LEVEL</span><span class="p">,</span> <span class="c"># &quot;alpha&quot;, &quot;beta&quot;, &quot;candidate&quot; or &quot;final&quot;; string</span>
 <span class="n">PY_RELEASE_SERIAL</span> <span class="c"># the 3; an int</span>
<span class="p">)</span>
</pre></div>
</div>
<p><em>OptionalRelease</em> records the first release in which the feature was accepted.</p>
<p>In the case of a <em>MandatoryRelease</em> that has not yet occurred,
<em>MandatoryRelease</em> predicts the release in which the feature will become part of
the language.</p>
<p>Else <em>MandatoryRelease</em> records when the feature became part of the language; in
releases at or after that, modules no longer need a future statement to use the
feature in question, but may continue to use such imports.</p>
<p><em>MandatoryRelease</em> may also be <tt class="xref docutils literal"><span class="pre">None</span></tt>, meaning that a planned feature got
dropped.</p>
<p>Instances of class <tt class="xref docutils literal"><span class="pre">_Feature</span></tt> have two corresponding methods,
<tt class="xref docutils literal"><span class="pre">getOptionalRelease()</span></tt> and <tt class="xref docutils literal"><span class="pre">getMandatoryRelease()</span></tt>.</p>
<p><em>CompilerFlag</em> is the (bitfield) flag that should be passed in the fourth
argument to the builtin function <a title="compile" class="reference external" href="functions.html#compile"><tt class="xref docutils literal"><span class="pre">compile()</span></tt></a> to enable the feature in
dynamically compiled code.  This flag is stored in the <tt class="xref docutils literal"><span class="pre">compiler_flag</span></tt>
attribute on <tt class="xref docutils literal"><span class="pre">_Feature</span></tt> instances.</p>
<p>No feature description will ever be deleted from <tt class="xref docutils literal"><span class="pre">__future__</span></tt>.</p>
</div>


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